1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, a printer, or a facsimile which carries out image formation using an electrophotographic method.
2. Description of the Related Art
In recent years, as image forming apparatuses have become increasingly energy-efficient, not only power reduction during operation and on standby but also power reduction during power-off and in energy-saving mode has become a very important issue as typified by ErP directive Lot 6 which is the European regulation.
Conventionally, in an ordinary arrangement of an input circuit that supplies commercial AC (alternating-current) power to an apparatus, a noise filter circuit placed on a commercial AC power line is disposed upstream of a power shutdown/energization device such as a relay (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. 2008-203880). In general, a noise filter circuit is comprised of a common mode choke coil, an X-capacitor, and a discharge resistor. A discharge resistor is intended to discharge residual electrical charge in the X-capacitor within a predetermined period of time specified by safety standards when a power plug is disconnected from a commercial AC power source. Thus, a discharge resistor is indispensable for a noise filter circuit, and dispensing with it is very difficult. An X-capacitor, which is a common name of an across-the-line capacitor, is placed across an AC line and intended to filter out noise. Depending on the capacity of an X-capacitor, a constant of about 100 kΩ to 500 kΩ is commonly selected as the resistance value of a discharge resistor. While commercial AC power is being supplied, electric current constantly flows through a discharge resistor, and hence a power loss caused by the discharge resistor occurs. When the resistance value of a discharge resistor lies inside the above range, and an input voltage is AC 200 V, a power loss of 0.08 W to 0.4 W caused by the discharge resistor occurs. This is not a negligible loss during power-off and in sleep mode when an apparatus is plugged in.
A discharge resistor is required so as to comply with a discharge time specified by safety standards as described above, and hence it is very difficult to increase resistance value more than is necessary or dispense with the discharge resistor itself. For this reason, a noise filter circuit is disposed downstream of a relay, and the relay is turned off during power-off and in sleep mode so as to inhibit electric current from flowing through a discharge resistor so that a power loss caused by the discharge resistor can be prevented.
However, if a noise filter circuit is disposed downstream of a relay, an X-capacitor as well should be inevitably disposed downstream of the relay, and when the relay is on, inrush current at the X-capacitor occurs. A relay contact reaches the end of its life when a surface condition thereof deteriorates, and contact sticking or poor contact occurs. In particular, a main factor that causes the surface of a relay contact to deteriorate is arc discharge occurring when a relay is turned and off. As the amount of inrush current increases, the amount of arc discharge occurring when the relay is turned on also increases.
Moreover, in recent years, automatic shifting into power-off and energy-saving mode has been required from the standpoint of energy conservation, and as a result, the number of times a relay is turned on and off is increasing. Under such circumstances, if a noise filter circuit is disposed downstream of a relay, inrush current will occur, and in addition, the number of times inrush current occurs will increase, causing a significant decrease in the life of a relay contact.